The biggest stars

it is not 100 percent sure but 200 solar masses looks like an upper bound on the mass
it is also not certain why.

if there is some upper limit, say between 130 and 200 solar mass,
then WHY?

why should stars be confined between this range, whatever it is (say 0.1 solar up to 150 solar).

one idea is that when a star is forming if it gets too big it makes so much light that the light drives away the cloud of material around the star that it is feeding on. I dont know if that is a current theory or not. maybe someone has read up on this

The central star in the Pistol Nebula is estimated to have a radius of about 93M miles (as big as Earth's orbit) and a total energy output 10M times that of the Sun. In this link, it is proposed that the star may have had an initial mass of nearly 250 times that of the Sun, but has been shedding material at a prodigious rate all its life.

It may be that (in suitable gas-rich regions of space) several hundred solar masses of material can agglomerate before the nuclear furnace at the core ignites and the new star starts driving away other neighboring material. In this model, the rapidity with which the material gathers would be an important constraint on initial mass. If the formative mass gathered very slowly, the star could ignite and form a strong stellar wind before it could get very massive.

I think the photon pressure is also important once the star has formed, in that it unsettles the hydrostatic equilbrium, this pressure is also sensitive to the optical depth, at depth in the star, and that is sensitive to the metallicity. Population III stars, the primordial giants that are supposed to have re-ionised the early IGM, would not have had as much/any metallicity(freely coasting or standard LCDM cosmology respectively) and therefore they may have been even larger.

The other reason such large stars are not observed is that their lifetimes are so short, you have to be quick to catch one before it goes supernova!